In an off-line, switching regulated voltage power supply for applications such as high-density CMOS logic, high-speed ECL logic, etc., the power dissipated in the output rectifiers accounts for at least 20-30 percent of the total input power. Losses from silicon rectifiers could be reduced by substitution of power Schottkty rectifiers. However, even with Schottky rectifiers, the power dissipated causes a significant reduction in power supply efficiency.
Synchronous rectifiers, although well known in low-power signal applications, have generally been impractical for power applications. Bipolar transistors have previously exhibited high saturation resistance, and required significant base drive current. Moreover, the reverse breakdown characteristics were unsuitable for power rectification. Furthermore, the storage time exhibited by previously available bipolar transistors was excessive for useful high-frequency switching regulators.
Alternately, a power MOSFET devices may be used as switching elements. However, the power MOSFET devices presently known have a reverse breakdown characteristic similar to that of a diode (temperature-dependent, high forward drop) and therefore unattractive for this application.